US envoy Kerry hails Syria ceasefire, says not enough

US Secretary of State John Kerry on Wednesday welcomed the Syrian army's declaration of a 72-hour ceasefire but said he was working with Russia and others to try to transform it into a lasting truce.

"We very much welcome the Syrian army's declaration of a period of quiet in honour and celebration of Eid," Kerry told reporters during an official visit to Georgia in the runup to the annual NATO summit.

"And we very much hope it will be honored by all parties and it will hold," he said, adding that the truce had been a "matter of discussion" within the International Syria Support Group, which Washington co-chairs with Moscow.

Earlier, the Syrian army had said it would observe a 72-hour pause in fighting between July 6 and midnight on July 8, to coincide with the festival marking the end of the Islamic holy month of Ramadan.

Syria has been in the grip of a bloody civil war for more than five years, but the 22-nation ISSG is pushing both government and rebel forces to honour a notional ceasefire and prepare for negotiations on a political settlement.

"We are engaged right now in ongoing discussions with various parties including Russia regarding the possibility of extending this," Kerry said, expressing the hope that the truce would be a "harbinger" of opportunities to come.

"So is 72 hours enough? The answer is simple: 'no'. Is 72 hours more welcome than nothing? The answer is 'yes'."